Trends in wireless communication technology continue towards ever higher data transfer rates in ever more tightly-packed frequency bands. Spectral efficiency in modern radio-frequency (RF) communication devices, e.g., cellular phones, tablet computers, laptop computers, etc., is achieved by adhering to stringent specifications on a number of system variables, many of which are interdependent. Not the least of these specifications is that on linearity of the RF power amplifier in the device's transmitter. Typically, such RF power amplifiers are operated at or near saturation and, consequently, considerable distortion is inflicted on the amplified signal. Amplitude modulation to amplitude modulation (AMAM) distortion results in gain compression that causes a higher bit error rate at the receiver. Amplitude modulation to phase modulation (AMPM) distortion results in constellation skew at the receiver that causes decoding/demodulation errors. It is thus increasingly common to predistort transmitter data in both amplitude and phase prior to amplification so that the amplified signal tracks linearly with the power amplifier's input signal.
Predistortion data are obtained through a calibration procedure and, since every power amplifier is slightly different, even between amplifiers of like design and manufacturing process, calibration is performed on every device prior to shipping from the manufacturer. Thus, considerable expenditure of design and engineering resources have sought, and continue to seek mechanisms by which calibration time can be shortened and/or by which calibration can be performed in the field. To that end, many manufacturers have begun to incorporate calibration circuitry into the communication device, including circuitry for test signal generation, signal probing and measuring, signal processing and analysis, parameter generation and process variable assignment, etc. Quite obviously, the added circuitry increases the device's complexity, size and cost. Accordingly, development of smaller, simpler, but nevertheless accurate calibration circuits and concomitant calibration techniques are ongoing.